1. Field of the Invention
This invention relates to a fuel feeding system for an internal combustion engine, and especially to a fuel injection system suitable for use in a cylinder injection internal combustion engine to permit injection of fuel at relatively high fuel pressure.
2. Description of the Related Art
As internal combustion engines of the type that fuel is injected within each cylinder, that is, as so-called cylinder injection internal combustion engines or direct injection internal combustion engines, diesel engines are known widely. In the field of spark ignition engines (hereinafter called "gasoline engine" because spark ignition engines are generally gasoline engines), those of the cylinder injection type have also been proposed in recent years.
In cylinder injection internal combustion engines, there is a tendency, with a view toward improving their performance and also reducing exhaust gas, to raise the fuel injection pressure so that fuel can be atomized as very fine droplets and the fuel injection cycle can be shortened. Further, engines equipped with a supercharging system require, upon supercharging, high fuel injection pressure corresponding to the pressure of the supercharging.
Fuel feeding systems in cylinder injection internal combustion engines are therefore constructed to successfully provide such high fuel injection pressure (for example, several tens atm or so).
For example, FIGS. 12 and 13 schematically illustrate the constructions of fuel feeding systems for internal combustion engines, which have heretofore been proposed as fuel feeding systems capable of providing such high fuel injection pressure.
Illustrated in FIGS. 12 and 13 are a fuel injector 1, a fuel tank 2, a fuel line 3 arranged between the fuel injector 1 and the fuel tank 2, a low-pressure fuel pump 4 arranged in an upstream portion of the fuel line 3, said upstream portion being on a side of the fuel tank 2, and a high-pressure fuel pump 5 arranged between the low-pressure fuel pump 4 and the fuel injector 1. Also depicted are fuel filters 6,7 arranged in an inlet portion of the fuel line 3, a check valve 8, a low-pressure regulator 9 as a low-pressure control means, and a high-pressure regulator 10 as a high-pressure control means. Further, FIG. 13 also shows a cylinder 21, a piston 22, a piston rod 22A, a crankshaft 23, a combustion chamber 24, a cylinder head 25, an intake passage 26, a spark plug 27, and an exhaust passage 28.
According to each of such fuel feeding systems, the fuel which has been pressurized to a certain extent by the low-pressure fuel pump 4 is pressurized further by the high-pressure fuel pump 5 so that the pressure of the fuel is raised to a predetermined level. Here, the delivery pressure from the low-pressure fuel pump 4 is stabilized within a predetermined range by the low-pressure regulator 9 while delivery pressure from the high-pressure fuel pump 4 is stabilized within another predetermined range by the high-pressure regulator 10.
The prior art in which fuel pressurized by such a low-pressure fuel pump is pressurized further by a high-pressure fuel pump prior to feeding it to each fuel injector include, for example, the technique disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 62-237057. According to this technique, a high fuel injection pressure can be provided in an operation range of high intake air pressure but in an operation range of low intake air pressure, the fuel injection pressure is held low to reduce the load on a high-pressure fuel pump.
Further, a technique on fuel injection control in a spark ignition, cylinder and direct injection internal combustion engine is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-183948. This technique is intended to increase the dispersibility of fuel and hence to promptly form a desired air-fuel mixture upon starting an engine at low temperatures by intermittently injecting the fuel in plural divided portions through an injector.
According to the technique disclosed in the above-referred Japanese Patent Application Laid-Open (Kokai) No. SHO 62-237057 referred to above, the pressure of fuel delivered from the high-pressure fuel pump is regulated by opening or closing a regulator not only when high-pressure fuel injection is needed but also when low-pressure fuel injection is needed. If the high-pressure fuel pump fails to operate fully, this high-pressure fuel pump interferes with force feeding by the low-pressure fuel pump, leading to the potential problem that the fuel pressure may not be regulated by the regulator to a fuel injection pressure corresponding to a desired operation range.
As the fuel pumps described above, it may be considered to adopt either engine-driven pumps or electric pumps. Adoption of an electric pump as a high-pressure pump however results in a lowered pump efficiency and higher manufacturing cost. It is therefore considered general to adopt an engine-driven pump as the high-pressure fuel pump. On the other hand, adoption of an electric pump as a low-pressure pump can make use of the merits of the electric pump that the above-described inconveniences in pump efficiency and manufacturing cost are reduced and stable delivery pressure can be obtained. It is hence considered reasonable to adopt an electric pump as the high-pressure pump.
However, the delivery pressure of an engine-driven pump is proportional to the engine speed. When an engine-driven pump is used as a high-pressure fuel pump, the delivery pressure of the high-pressure fuel pump is extremely low upon start-up of the engine because the engine speed is low at that time. The high-pressure fuel pump interposed between the low-pressure fuel pump and the fuel injector therefore conversely interferes with the flow of fuel so that the fuel pressure at the fuel injector may not reach even the level of a delivery pressure from the low-pressure fuel pump.
The technique disclosed in the above-referred Japanese Patent Application Laid-Open (Kokai) No. SHO 62-237057 therefore involves the potential problem that the fuel injection pressure of the fuel injector controlled by the regulator may not be raised a necessary level at the time of start-up of the engine and may not be controlled according to the operation range.
Further, the delivery pressure of an engine-driven pump is proportional to the engine speed. When an engine-driven pump is used as the high-pressure fuel pump, the delivery rate of the high-pressure fuel pump is substantially equal to the injection rate of fuel at low engine speeds but when the engine speed increases, the volumetric efficiency of the high-pressure fuel pump increases and its delivery rate may become too much. In the case of a low-pressure fuel pump making use of an electric pump, on the other hand, the delivery rate does not depend on the engine speed, and the pump is generally set at a delivery rate somewhat greater than a maximum fuel injection quantity, that is, (the maximum fuel injection quantity+.alpha., .alpha.&gt;0) so that the maximum fuel injection quantity can be assured.
Accordingly, the delivery rate of the high-pressure fuel pump becomes greater than that of the low-pressure fuel pump at a high engine speed, leading to the potential problem that the pressure of fuel at the inlet of the high-pressure fuel pump may drop and cavitation may occur there.
Such cavitation is not preferred for the injection of fuel and moreover, may damage the pump, fuel line and injector.
In an automotive internal combustion engine, for example, vapor (bubbles) tends to occur in the fuel line 3 due to a temperature increase in an engine compartment (bay) as a result of inactivation of an engine cooling system after the engine is stopped and/or due to leakage of fuel at the pressure regulators 9,10 and the fuel injector 1. When the driver subsequently attempts to start the engine, feeding of fuel is started with the vapor contained in the fuel line 3.
In the conventional multipoint injection (MPI) or the like, the pressure of fuel fed to the fuel injector 1 is not very high and a pressure regulator arranged downstream the fuel injector 1, said pressure regulator corresponding to the pressure regulator 10 in the above-described fuel feeding system, or the like has a low preset pressure. The fuel pressure promptly reaches this preset pressure subsequent to start-up of the engine so that the fuel inside the fuel line 3 on a side of the fuel injector 1 is allowed to flow out of the pressure regulator in a short time. The vapor in the fuel is therefore caused to flow away along with the fuel.
However, when the pressure of the fuel to be fed to the fuel injector 1 is substantially higher than that in the conventional multipoint injection (MPI) or the like, the preset pressure of the pressure regulator 10 is high. Until the fuel pressure reaches this preset pressure, the fuel in the fuel line 3 on the side of the fuel injector 1 is not allowed to flow out of the pressure regulator in a short time. The vapor in the fuel therefore remains together with the fuel on the side of the fuel injector 1 and accordingly, is not caused to flow away promptly.
Incidentally, it is generally common to any pump that a certain time is needed until the fuel pressure reaches a high preset pressure. In a fuel feeding system of such a construction as described above, however, it takes a substantial time for the below-described reason until the fuel pressure increases to a high level.
It may be considered to employ either an engine-driven pump or an electric pump as each of the fuel pumps 4,5. Use of an electric pump as the high-pressure pump however leads to lower pump efficiency and higher manufacturing cost. It is therefore considered common to employ an engine-driven pump as the high-pressure fuel pump 5. Use of an electric pump as a low-pressure pump can reduce the above-mentioned inconvenience in pump efficiency and manufacturing cost and can make use of the advantage of an electric pump that a stable delivery pressure can be obtained. It is therefore considered to employ an electric pump as the low-pressure fuel pump 4.
Since the delivery pressure of an engine-driven pump is proportional to the engine speed, use of the engine-driven pump as the high-pressure fuel pump 5 however results in a low engine speed and hence an extremely low delivery pressure of the high-pressure pump at the time of start-up of the engine so that the fuel pressure does not rise promptly.
The vapor in the fuel therefore is not caused to promptly flow out of the fuel injector 1. Such vapor, which remains around the fuel injector 1, leads to a delay or scattering in the rise of the pressure of the fuel to be injected or causes injection without fuel from the fuel injector 1, so that the control of the fuel injection is rendered difficult. It is therefore not preferred to use an engine-driven pump as the high-pressure fuel pump 5.
Taking elimination of such vapor from fuel in a broad sense, the related art is found, for example, in Japanese Utility Model Application Laid-Open (Kokai) NO. HEI 4-107474 which discloses the provision of an air vent hole in a fuel line immediately downstream a fuel filter. Further, Japanese Utility Model Application Laid-Open (Kokai) No. HEI 2-145657, for example, discloses a fuel feeding system which comprises a fuel injection pump and also an air-venting fuel pump arranged in series with the fuel injection pump. Also arranged is a bypass fuel line which circumvents the fuel injection pump. A check valve is arranged in the bypass line so that fuel is allowed to flow only in a direction from the side of a fuel injector toward the air-venting fuel pump.
None of these techniques is however directed to a fuel feeding system which is provided with a high-pressure regulator on a downstream side of a fuel injector and features high fuel pressure, so that it is difficult to eliminate the above-mentioned vapor from fuel present at the fuel injector 1.